Parkinson's disease (PD) is the second most common neurodegenerative disorder, afflicting as many as one million Americans. The core psychomotor symptoms of the disease are attributable to the degeneration of dopaminergic neurons innervating the striatum. Treatment strategies for PD patients are limited, making it imperative that we gain a better understanding of how DA - and its loss in PD - shapes striatal function. There are profound experimental obstacles that have prevented us from gaining the kind of conceptual foothold we need to make real a translational impact for PD patients. Two critical obstacles are striatal cellular heterogeneity and the inaccessibility of the small, spiny dendrites of MSNs - the principal site of DA modulation - to experimental interrogation. In the last grant period, we have developed approaches necessary to overcome these obstacles. To overcome cellular heterogeneity, we have taken advantage of Bacterial Artificial Chromosome (BAC) transgenic mice in which D1 or D2 receptor-expressing MSNs are labeled with enhanced green fluorescent protein (eGFP). These mice have enabled electrophysiological study of the functional properties of identified MSNs in tissue slices, providing unprecedented insights into how these two principal cell types differ and adapt in PD models. To overcome the inaccessibility of MSN dendrites to conventional patch clamp techniques, we have gained expertise in two- photon laser scanning microscopy (2PLSM) and 2P laser uncaging (2PLU), which make even the fine distal dendrites of MSNs accessible to structural and functional study, creating a window into the striatal adaptations in PD. The studies proposed in this revised renewal application marshal these and other powerful new tools to attack fundamental gaps in our understanding of the striatal pathophysiology in PD. We propose three specific aims that build upon the results obtained in the last funding period: 1) To characterize intrinsic ionic mechanisms governing dendritic excitability and synaptic integration in striatal medium spiny neurons (MSNs). Our central hypothesis is that MSN dendrites are invested with Ca2+ and K+ channels that govern short-term processing of synaptic inputs and long-term changes in synaptic strength; 2) To characterize the signaling mechanisms underlying the induction and expression of synaptic plasticity in MSNs. Our central hypothesis is that DA receptor signaling cascades are critical to the induction of bidirectional, Hebbian plasticity at corticostriatal glutamatergic synapses; 3) To characterize MSN somatodendritic adaptations in animal models of Parkinson's disease. Our central hypothesis is that dysregulation of dendritic excitability and synaptic plasticity following the loss of DA triggers cell-type homeostatic adaptations in MSNs that are responsible for pathological activity patterns and motor deficits in PD. The insights gained from this work should promote the development of novel and powerful therapeutic strategies for PD.